Optical pickup and tilt control device including a plurality of photosensitive units for generating signals indicating whether tilt correction is necessary

ABSTRACT

An optical pickup for reading information recorded on a recording surface of an optical information storage medium has a light beam source for emitting a linear light beam, and an optical system including an objective lens for converging the linear light beam as a linear image on the recording surface and collecting and emitting a light beam reflected from the recording surface. A tilt control device includes a parallel flat glass plate for refracting the reflected light beam from the optical system, a plurality of photodetector units each having a plurality of photosensitive surfaces for photoelectrically converting the light beam applied thereto from the parallel flat glass plate into a plurality of respective detected signals, an error generator for processing the detected signals from the photodetector units into a tilt error signal indicative of whether the linear light beam is applied perpendicularly to the recording surface, and an actuator responsive to the tilt error signal for controlling the objective lens positionally with respect to the optical information storage medium to apply the linear light beam perpendicularly to the recording surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup for reading recordedinformation from an optical information storage medium such as a compactdisc, a laser video disc, or the like, and a tilt control device for usewith such an optical pickup.

2. Description of the Prior Art

Some optical pickups for reading recorded information from an opticalinformation storage medium such as a compact disc, a laser video disc,or the like have a tilt detecting device for detecting whether a lightbeam emitted from a light source is applied perpendicularly to therecording surface of the optical information storage medium.

One tilt detecting device for use with an optical pickup is shown inFIGS. 1(A) and 1(B) of the accompanying drawings. As shown in FIGS. 1(A)and 1(B), the tilt detecting device, generally designated by thereference numeral 200, has a semiconductor laser 31 for emitting a laserbeam toward an optical disc DK, two photodetectors 32, 33 for detectingand photoelectrically converting a light beam reflected from a recordingsurface S₂ of the optical disc DK into detected electric signals, and adifferential amplifier 34 for calculating the difference between thedetected signals from the photodetectors 32, 33 to produce a tilt errorsignal TE indicative of whether the laser beam is appliedperpendicularly to the recording surface S₂ of the optical disc DK.

If the semiconductor laser 31 and the photodetectors 32, 33 faceproperly to the optical disc DK, as shown in FIG. 1(A), then the twophotodetectors 32, 33 detect equal intensities of the reflected lightbeam, and hence the differential amplifier 34 produces output signal ofzero. If the optical disc DK is tilted with respect to the semiconductorlaser 31 and the photodetectors 32, 33 as shown in FIG. l(B), thephotodetector 32 detects a higher intensity of the reflected light beamthan the photodetector 33, so that the differential amplifier 34produces a positive output signal, thus detecting the tilt of theoptical disc DK. A suitable control unit including an actuator may beconnected to the tilt detecting device 100, thus making up a tiltcontrol device for correcting the position of the optical pickup so thatthe laser beam will be applied perpendicularly to the recording surfaceS₂ of the optical disc DK.

The tilt control device is disadvantageous in that its semiconductorlaser 31 and the photodetectors 32, 33 are required in addition to theoptical pickup which is used to reproduce an information signal recordedon the optical disc DK. Therefore, the entire assembly of the opticalpickup and the tilt control device has a complex structure and a largesize.

SUMMARY OF THE INVENTION

It is an object, of the present invention to provide an optical pickupwhich is relatively simple in structure.

Another object of the present invention is to provide a tilt controldevice of a relatively simple structure for an optical pickup.

According to the present invention, there is provided an optical pickupfor reading information recorded on a recording surface of an opticalinformation storage medium, including a light beam source for emitting alinear light beam, an optical system for converging the linear lightbeam as a linear image on the recording surface and collecting andemitting a light beam reflected from the recording surface, refractingunit for refracting the reflected light beam emitted from the opticalsystem, and a plurality of photodetector units each having a pluralityof photosensitive surfaces for photoelectrically converting the lightbeam applied thereto from the refracting unit into a plurality ofrespective detected signals indicative of a tilt of the optical systemwith respect to the optical information storage medium.

According to the present invention, there is also provided a tiltcontrol device for use in an optical pickup for reading informationrecorded on a recording surface of an optical information storagemedium, including a light beam source for emitting a linear light beam,an optical system including an objective lens for converging the linearlight beam as a linear image on the recording surface and collecting andemitting a light beam reflected from the recording surface, refractingunit for refracting the reflected light beam emitted from the opticalsystem, a plurality of photodetector units each having a plurality ofphotosensitive surfaces for photoelectrically converting the light beamapplied thereto from the refracting unit into a plurality of respectivedetected signals, processing unit for processing the detected signalsfrom the photodetector units into a tilt error signal indicative ofwhether the linear light beam is applied perpendicularly to therecording surface, and control unit responsive to the tilt error signalfor controlling the objective lens positionally with respect to theoptical information storage medium to apply the linear light beamperpendicularly to the recording surface.

The reflected light beam from the optical system is refracted by therefracting unit and then applied to the photosensitive surfaces of thephotodetector units. The detected signals from the photodetector unitsare processed by the processing unit into the tilt error signalindicative of whether each of the photodetector units is too close to orfar from the recording surface of the optical information storagemedium. The photosensitive surfaces of the photodetector units canproduce the detected signals using a portion of the reflected light beamapplied thereto. The photodetector units may also be used to detect afocus error signal. Therefore, the optical pickup may be relativelysimple in structure.

The above and other objects, features, and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) and 1(B) are schematic cross-sectional views of a tiltdetecting device;

FIG. 2 is a schematic view of a tilt control device according to a firstembodiment of the present invention;

FIG. 3 is a diagram showing the relationship between signal pits and alinear beam spot on an optical disc for the tilt control deviceaccording to the first embodiment;

FIGS. 4(A) and 4(B) are front elevational and plan views, respectively,of a photodetector in the tilt control device according to the firstembodiment;

FIGS. 4(C) and 4(D) are side elevational views of the photodetector, asviewed in the directions indicated by the arrows I and II, respectively,in FIG. 4(A);

FIGS. 5(A), 5(B), and 5(C) are side elevational views showing a linearbeam applied to the photodetector shown in FIGS. 4(A) and 4(B);

FIG. 6 is a schematic view, partly in block form, of the photodetectorand an error generator in the tilt control device according to the firstembodiment;

FIGS. 7(A), 7(B), and 7(C) are schematic views showing the manner inwhich the tilt control device according to the first embodimentoperates;

FIGS. 8(A), B(B), and 8(C) are schematic views showing the manner inwhich the tilt control device according to the first embodimentoperates;

FIG. 9 is a plan view of a photodetector in a tilt control deviceaccording to a second embodiment of the present invention;

FIGS. 10(A) and 10(B) are front elevational and plan views,respectively, of a photodetector in a tilt control device according to athird embodiment of the present invention;

FIGS. 11(A) and 11(B) are front elevational and plan views,respectively, of a photodetector in a tilt control device according to afourth embodiment of the present invention;

FIG. 11(C) is a cross-sectional view taken along line III--III of FIG.11(A);

FIGS. 11(D) and 11(E) are side elevational views of the photodetector,as viewed in the directions indicated by the arrows IV and V,respectively, in FIG. 11(A);

FIG. 12(A) is a diagram showing the relationship between signal pits anda linear beam spot on an optical disc for a tilt control deviceaccording to a fifth embodiment of the present invention;

FIG. 12(B) is a plan view of a photodetector for use in the tilt controldevice according to the fifth embodiment of the present invention; and

FIG. 13 is a plan view of a photodetector for use in a tilt controldevice according to a sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1st Embodiment

FIGS. 2 through 8(A), 8(B), 8(C) show a tilt control device for use withan optical pickup, according to a first embodiment of the presentinvention. As shown in FIG. 2, the tilt control device, generallydesignated by the reference numeral 100, has a semiconductor laser I foremitting a linear laser beam having a predetermined length across itsoptical axis, a beam splitter 2 having a reflecting surface S, forreflecting the laser beam from the semiconductor laser 1 toward anoptical disc DK as an optical information storage medium, a collimatorlens 3 for converting the laser beam from the beam splitter 2 into aparallel laser beam, and an objective lens 4 for converging the parallellaser beam from the collimator lens 3 as a linear image or a linear beamspot on a recording surface S₂ of the optical disc DK.

The tilt control device 100 also has a photodetector 5A for detecting aportion of a linear beam which has been reflected by the recordingsurface S₂, traveled back through the objective lens 4 and thecollimator lens 3, passed through the reflecting surface S, in the beamsplitter S₂, and reached a surface S₃, and photoelectrically convertingthe detected linear beam portion into a plurality of detected electricsignals, an error generator 6A for generating a tilt error signal TEindicating whether the optical pickup, namely, the objective lens 4, istilted with respect to the optical disc DK, based on the detectedelectric signals from the photodetector 5A, a driver 7 for producing adrive signal based on the tilt error signal TE from the error generator7, and an actuator 8 responsive to the drive signal for controlling thetilt of the objective lens 4 with respect to the optical disc DK.

Another portion of the linear beam that has fallen on the surface S₃ isread as an RF signal indicative of the information recorded on theoptical disc DK, and the RF signal is sent to an RF amplifier 9. The RFsignal is amplified at a suitable gain by the RF amplifier 9, whichapplies the amplified RF signal to a signal demodulator (not shown).

FIG. 3 shows the relationship between signal pits SP and a linear beamspot LS on the optical disk DK, the linear beam spot LS being formed onthe recording surface S₂ by the laser beam applied thereto. The linearbeam spot LS extends linearly perpendicularly to a track direction,i.e., a circumferential direction, and covers only one track or row ofsignal pits SP but does not reach adjacent tracks.

FIGS. 4(A), 4(B), 4(C), and 4(D) show the photodetector 5A in detail. Asshown in FIGS. 4(A), 4(B), 4(C), and 4(D), the photodetector 5A includesa photodetector element 13 for detecting a linear beam spot LS reflectedfrom the optical disc DK and two parallel flat glass plates 11, 12mounted on and covering portions of the photodetector element 13. Thephotodetector element 13 has three photosensitive surfaces a₁, a₂, a₃successively arranged across the photodetector element 13 at one endthereof, a single photosensitive surface a₄ extending across thephotodetector element 13 and spaced from the three photosensitivesurfaces a₁, a₂, a₃, and three photosensitive surfaces a₅, a₆, a₇successively arranged across the photodetector element 13 at theopposite end thereof and spaced from the photosensitive surface a₄. Theparallel flat glass plates 11, 12 cover the three photosensitivesurfaces a₁, a₂, a₃ and a₅, a₆, a₇ in their entirety. Each of theparallel flat glass plates 11, 12 has a thickness d₁ and an absoluterefractive index n₁ and an optical path n₁ ×d₁. The photosensitivesurfaces a₁ -a₇ photoelectrically convert the detected reflected linearbeam spot LS into respective detected electric signals I₁ -I₇. Thesignals I₁ -I₃, I₅ -I₇ are used to detect a tilt error, and the signalI₄ represents an information signal indicative of the informationrecorded on the optical disc DK. As shown in FIG. 4(B), the reflectedlinear beam spot LS from the optical disc DK is applied across thephotosensitive surfaces a₁ -a₇.

Operation of the tilt control device according to the first embodimentwill be described below with reference to FIGS. 5(A), 5(B), 5(C) throughB(A), 8(B), 8(C).

FIGS. 5(A), 5(B), 5(C) show the reflected linear beam LS that is appliedto the photodetector 5A, as viewed in the direction indicated by thearrow I in FIG. 4(A). In FIG. 5(A), the reflected linear beam spot LS isshown as being applied to the photodetector 5A when the distance betweena photodetector unit 13₁ composed of the photosensitive surfaces a₁ -a₃and the optical disc DK is shorter than the focal length of theobjective lens 4, i.e. , when the photodetector unit 13₁ is too close tothe optical disc DK. In FIG. 5(B), the reflected linear beam spot LS isshown as being applied to the photodetector 5A when the distance betweenthe photodetector unit 13₁ and the optical disc DK is equal to the focallength of the objective lens 4. In FIG. 5(C), the reflected linear beamspot LS is shown as being applied to the photodetector 5A when thedistance between the photodetector unit 13₁ and the optical disc DK islonger than the focal length of the objective lens 4, i.e., when thephotodetector unit 13₁ is too far from the optical disc DK. In FIGS.5(A), 5(B), and 5(C), the reflected linear beam LS applied to thephotodetector 5A travels through the parallel flat glass plate 11 alonga trajectory indicated by the solid-line arrows, and travels outside ofthe parallel flat glass plate 11 along a trajectory indicated by thebrokenline arrows. The reflected linear beam LS has widths b₁, b₂, b₃when they are applied as shown in FIGS. 5(A), 5(B), and 5(C),respectively, and these widths b₁, b₂, b₃ satisfy the relationship b₁<b₂ <b₃ because of the refraction by the parallel flat glass plate 11.The above process holds true for the reflected linear beam LS which isapplied through the parallel flat glass plate 12 to a photodetector unit13₂ composed of the photosensitive surfaces a₅ -a₇.

FIG. 6 shows the photodetector 5A and the error generator 6A connectedto the photodetector 5A. As shown in FIG. 6, the error generator 6A hastwo adders 23, 24 and three subtractors 25, 26, 27. The adder 23 has oneinput terminal connected to the photosensitive surface a₁ and the otherinput terminal connected to the photosensitive surface a₃. Thesubtractor 25 has a negative input terminal connected to thephotosensitive surface a₂ and a positive input terminal connected to theoutput terminal of the adder 23. The adder 24 has one input terminalconnected to the photosensitive surface a₅ and the other input terminalconnected to the photosensitive surface a₇. The subtractor 26 has anegative input terminal connected to the photosensitive surface a₆ and apositive input terminal connected to the output terminal of the adder24. The subtractor 27 has a positive input terminal connected to theoutput terminal of the subtractor 25 and a negative input terminalconnected to the output terminal of the subtractor 26. Therefore, theerror generator 6A produces an output signal TE indicated by:

    TE=(I.sub.1 +I.sub.3 -I.sub.2)-(I.sub.5 +I.sub.7 -I.sub.6) (1)

as an output signal from the output terminal of the subtractor 27.

If the distance between the photodetector unit 13₁ and the optical discDK and the distance between the photodetector unit 13₂ and the opticaldisc DK are the same as each other, then the reflected laser beam spotLS is applied to the photodetector 5A as shown in FIGS. 7(A), 7(B), and7(C). The reflected laser beam spot LS has a width b₄ on thephotosensitive surfaces of each of the photodetector units 13₁, 13₂.Therefore, the output signal from the subtractor 25 and the outputsignal from the subtractor 26 are equal to each other, and the outputsignal TE of the error generator 6A is TE =0.

If the recording surface S₂ of the optical disc DK is inclined withrespect to the linear laser beam applied thereto, e.g., if thephotodetector unit 13₁ is farther from the optical disc DK and thephotodetector unit 13₂ is closer to the optical disc DK, then thereflected laser beam spot LS is applied to the photodetector 5A as shownin FIGS. B(A), 8(B), and 8(C). The reflected laser beam spot LS has awidth b₅ on the photosensitive surfaces of the photodetector unit 13₁,and the reflected laser beam spot LS has a width b₆ on thephotosensitive surfaces of the photodetector unit 13₂, with the width b₅being larger than the width b₆ (b₅ >b₆).

The photosensitive surfaces and the optical system are designed suchthat when the linear laser beam spot or linear image on the recordingsurface S₂ of the optical disc DK is focused at the focal point of theobjective lens 4, the respective detected signals I₁, I₂, I₃, I₅, I₆, I₇satisfy the following equations:

    FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 =0                     (2)

and

    FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 =0                     (3).

Then, when either one of the photodetector units 13₁, 13₂ is closer tothe optical disc DK, as shown in FIG. 5(A), because of the refraction bythe parallel flat glass plate 11 or 12, the detected signal I₂ from thephotosensitive surface a₂ is larger than the sum of the detected signalsI₁, I₃ from the photosensitive surfaces a₁, a₃. As a result, thefollowing relationship is satisfied:

    FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 <0                     (4)

and similarly

    FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 <0                     (5).

Conversely, when either one of the photodetector units 13₁, 13₂ fartherfrom the optical disc DK, as shown in FIG. 5(C), because of therefraction by the parallel flat glass plate 11 or 12, the detectedsignal I₂ from the photosensitive surface a₂ is smaller than the sum ofthe detected signals I₁, I₃ from the photosensitive surfaces a₁, a₃. Asa result, the following relationship is satisfied:

    FE.sub.1 =I.sub.1 +I.sub.3 -I.sub.2 >0                     (6)

and similarly

    FE.sub.2 =I.sub.5 +I.sub.7 -I.sub.6 >0                     (7).

When the reflected linear beam spot LS is applied to the photodetector5A as shown in FIGS. 8(A) through 8(C), therefore, the output signalfrom the error generator 6A is indicated by:

    TE=(I.sub.1 +I.sub.3 -I.sub.2)-(I.sub.5 +I.sub.7 -I.sub.6)>0(8).

When the photodetector unit 13₁ is closer to the optical disc DK and thephotodetector unit 13₂ LS farther from to the optical disc DK, theoutput signal from the error generator 6A is indicated by:

    TE<0                                                       (9).

Accordingly, the output signal from the error generator 6A can beemployed as a tilt error signal.

This is because the parallel flat glass plates 11, 12 each having apredetermined optical path and a refractive action cover the entirephotosensitive surfaces a₁, a₂, a₃ and a₅, a₆, a₇ causing the reflectedlinear beam spot LS to have widths b₁, b₂, b₃ (b₁ <b₂ <b₃),respectively, on the photosensitive surfaces when the optical disc DK isinclined and not inclined with respect to the objective lens 4. If thephotosensitive surfaces were not covered with the parallel flat glassplates 11, 12, then the reflected linear beam spot LS would fall on thephotosensitive surfaces along the trajectory as indicated by the brokenlines in FIGS. 4(A), 4(B), and 4(C). Although the output signals FE₁,FE₂ are FE₁ =0, FE₂ =0 when the laser beam spot is focused on theoptical disc DK, the output signals FE₁, FE₂ would be FE₁ >0, FE₂ >0when the laser beam spot is out of focus on the optical disc DKirrespective of whether it is overfocused or underfocused. Therefore, itwould be impossible to determine whether either one of the photodetectorunits 13₁, 13₂ is closer to or farther from the optical disc DK, thusfailing to determine which direction the optical disc DK is tilted inwith respect to the objective lens 4 from the tilt error signal TE.

2nd Embodiment

FIG. 9 shows a tilt control device according to a second embodiment ofthe present invention. The tilt control device according to the secondembodiment is similar to the tilt control device according to the firstembodiment except for a photodetector and an error generator. Therefore,only a photodetector and an error generator in the tilt control deviceaccording to the second embodiment are shown in FIG. 9.

As shown in FIG. 9, the photodetector, generally denoted at 5B, includesa photodetector element 14 for detecting a linear beam reflected fromthe optical disc DK and two parallel flat glass plates (not shown)mounted on and covering photodetector units 14₁, 14₂ in their entirety,of the photodetector element 14. The photodetector unit 14₁ is composedof three photosensitive surfaces a₈, a₉, a₁₀ successively arrangedacross the photodetector element 14 at one end thereof. A singlephotosensitive surface a₁₁ extends across the photodetector element 14and is spaced from the photosensitive surfaces a₈, a₉, a₁₀. Thephotodetector unit 142 is composed of three photosensitive surfaces a₁₂,a₁₃, a₁₄ successively arranged across the photodetector element 14 atthe other end thereof and spaced from the photosensitive surface a₁₁.The central photosensitive surfaces a₉, a₁₃ of the photodetector units14₁, 14₂, which detect a central portion of the reflected linear beamspot LS applied thereto, are connected to positive and negative inputterminals of an error generator 6B. The error generator 6B generates anoutput signal indicative of the difference between output signals fromthe central photosensitive surfaces a₉, a₁₃, as follows:

    TE=I.sub.9 -I.sub.13                                       (10)

In FIG. 9, the photodetector unit 14₁ is farther from the optical discDK than the photodetector unit 14₂. Since the reflected beam spot LS isdiffused as a whole, the relationship I₉ <I₁₃ is satisfied. Therefore,

    TE<0                                                       (11).

Conversely, when the photodetector unit 14₁ is closer to the opticaldisc DK than the photodetector unit 14₂,

    TE>0                                                       (12).

Accordingly, the output signal TE from the error generator 6B can beemployed as a tilt error signal.

3rd Embodiment

FIGS. 10(A) and 10(B) show a tilt control device according to a thirdembodiment of the present invention. The tilt control device accordingto the third embodiment is similar to the tilt control device accordingto the first embodiment except for a photodetector and an errorgenerator.

As shown in FIGS. 10(A) and 10(B), the photodetector, generally denotedat 5C, includes a photodetector element 17 and two parallel flat glassplates 15, 16 mounted on and covering photodetector units 17₁, 17₂, intheir entirety, of the photodetector element 17. The photodetector unit17₁ is composed of two parallel photosensitive surfaces a₁₆, a₁₇extending across the photodetector element 17 at one end thereof. Asingle photosensitive surface a₁₈ extends across the photodetectorelement 17 and is spaced from the photosensitive surfaces a₁₇, a₁₉. Thephotodetector unit 17₂ is composed of two parallel photosensitivesurfaces a₁₉, a₂₀ extending across the photodetector element 17 at theother end thereof and spaced from the photosensitive surface a₁₈. Eachof the parallel flat glass plates 15, 16 has a thickness d₂ and anabsolute refractive index n₂ and an optical path n₂ ×d₂. Thephotosensitive surfaces a₁₆ -a₂₀ photoelectrically convert the detectedreflected linear beam spot LS into respective detected electric signalsI₁₆ - I₂₀. The signals I₁₆, I₁₇, I₁₉, I₂₀ are used to detect a tilterror, and the signal I₁₈ represents an information signal indicative ofthe information recorded on the optical disc DK. As shown in FIG. 10(B),the reflected linear beam spot LS from the optical disc DK is appliedacross the photosensitive surfaces a₁₇ -a₁₉, and has opposite endsfalling on the photosensitive surfaces a₁₆, a₂₀, respectively.

Operation of the tilt control device according to the third embodimentwill be described below with reference to FIG. 10(B).

When the distance between the optical disc DK and the objective lens isproper, the photosensitive surfaces a₁₆, a₁₇, a₁₉, a₂₀ produce theiroutput signals which meet the following equations:

    FE.sub.1 =I.sub.16 -I.sub.17 0                             (13)

and

    FE.sub.2 =I.sub.19 -I.sub.20 =0                            (14)

and

    TE=FE.sub.1 -FE.sub.2 =0                                   (15)

where TE is the output signal from the error generator.

When the linear laser beam is not applied perpendicularly to the opticaldisc DK, the output signal TE becomes TE>0 or TE<0 because of therefraction by the parallel flat glass plate 15 or 16. The tilt of theobjective lens with respect to the optical disc DK can be controlledbased on the output signal or tilt error signal TE.

4th Embodiment

FIGS. 11(A) through 11(E) show a tilt control device according to afourth embodiment of the present invention. The tilt control deviceaccording to the fourth embodiment is similar to the tilt control deviceaccording to the first embodiment except for a photodetector and anerror generator.

As shown in FIGS. 11(A) and 11(B), the photodetector, generally denotedat 5D, includes a photodetector element 19 and a parallel flat glassplate 18 mounted on and covering a portion of the photodetector element19. The photodetector element 19 has a photodetector unit 19, composedof three photosensitive surfaces a₂₁, a₂₂, a₂₃ successively arrangedacross the photodetector element 19 at one end thereof, a singlephotosensitive surface a₂₄ extending across the photodetector element 19and spaced from the photodetector unit 19₁, and a photodetector unit 19₂composed of three photosensitive surfaces a₂₅, a₂₆, a₂₇ successivelyarranged across the photodetector element 19 at the other end thereofand spaced from the photosensitive surface a₂₄. The centralphotosensitive surface a₂₄ is covered in its entirety by the parallelflat glass plate 18. The parallel flat glass plate 18 has a thickness d₃and an absolute refractive index n₃ and an optical path n₃ ×d₃. Thephotosensitive surfaces a₂₁ -a₂₇ photoelectrically convert the detectedreflected linear beam spot LS into respective detected electric signalsI₂₁ -I₂₇. The signals I₂₁ -I₂₃, I₂₅ -I₂₇ are used to detect a tilterror, and the signal I₂₄ represents an information signal indicative ofthe information recorded on the optical disc DK. As shown in FIG. 11(B),the reflected linear beam spot LS from the optical disc DK is appliedacross the photosensitive surfaces a₂₁ -a₂₇. The central photosensitivesurfaces a₂₂, a₂₆ of the photodetector units 19₁, 19₂ are connected topositive and negative input terminals of an error generator 6D. Theerror generator 6D generates an output signal indicative of thedifference between output signals from the central photosensitivesurfaces a₂₂, a₂₆, as follows:

    TE=I.sub.22 -I.sub.26                                      (26).

The reflected beam spot LS is applied to the photodetector 5D along thetrajectory indicated by the solidline arrows in FIGS. 11(C) through11(E). The output signal TE from the error generator 6D can be employedas a tilt error signal.

5th Embodiment

In each of the first through fourth embodiments described above, thelinear beam spot LS is applied to the optical disc DK so that it doesnot fall on adjacent tracks at the same time, as shown in FIG. 3.According to a fifth embodiment, however, a longer linear beam spot LSis applied to the optical disc across a plurality of tracks of signalpits SP, as shown in FIG. 12 (A) . According to the fifth embodiment, asshown in FIG. 12(B), a photodetector 5E has a photosensitive surface a₃₁for reading an information signal recorded on the optical disc DK, aphotodetector unit 20₁ composed of three photosensitive surfaces a₂₈,a₂₉, a₃₀ spaced from the photosensitive surface a₃₁, and a photodetectorunit 20₂ composed of three photosensitive surfaces a₃₂, a₃₃, a₃₄ spacedfrom the photosensitive surface a₃₁. The three photosensitive surfacesa₂₈, a₂₉, a₃₀ and the three photosensitive surfaces a32, a₃₃, a₃₄ arecovered, in their entirety, with respective parallel flat glass plates(not shown) as with the first embodiment. The photodetector units 20₁,20₂ jointly serve to produce a tilt error signal. If laser beamreflections from other tracks than those tracks from which a tilt errorsignal is to be derived are also applied to the photodetector unit 20₁or 20₂, then the photodetector unit 20₁ or 20₂ may be arranged such thatit cannot detect signals in a high-frequency range.

6th Embodiment

As shown in FIG. 13, a photodetector 5F according to a sixth embodimentof the present invention has a plurality of juxtaposed photosensitivesurfaces 22₁ -22_(n) for reading information signals from the opticaldisc DK using a very long linear beam spot LS applied to the opticaldisc DK. The photodetector 5F is thus capable of reading a plurality oftracks on the optical disc DK at the same time. The photodetector 5Falso has a photodetector unit 21₁ composed of three photosensitivesurfaces a₃₅, a₃₆, a₃₇, and a photodetector unit 21₂ composed of threephotosensitive surfaces a₃₈, a₃₉, a₄₀ spaced from the photodetector unit21₁. The photodetector units 21₁, 21₂ are covered in their entirety withrespective parallel flat glass plates (not shown) as with the firstembodiment.

In the above embodiments, certain photosensitive surfaces of thephotodetector are covered with a parallel flat glass plate. However,they may be covered with a lens, a Fresnel lens, a cylindrical lens, oran optical element with a varying distribution of refractive indexes.Such a lens or an optical element may be spaced from the photosensitivesurfaces.

In the illustrated embodiments, the photosensitive surfaces forproducing a tilt error signal and an information signal are disposed inthe same plane. However, the photosensitive surface for producing aninformation signal may be positioned somewhere else, and a reflectedlight beam may be guided thereto by a half-silvered mirror or some otheroptical element.

While each of the illustrated photodetectors is shown as including twophotodetector units, it may be composed of three or more photodetectorunits.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

What is claimed is:
 1. An optical pickup for reading informationrecorded on a recording surface of an optical information storagemedium, comprising:a light beam source for emitting a linear light beam;an optical system for converging said linear light beam as a linearimage on the recording surface and collecting and emitting a linearlight beam reflected from said recording surface; refracting means forrefracting the reflected linear light beam emitted from said opticalsystem; and a plurality of photodetector units each having a pluralityof photosensitive surfaces for photoelectrically converting thereflected linear light beam applied thereto from said refracting meansinto a plurality of respective detected signals indicative of a tilt ofsaid optical system with respect to the optical information storagemedium.
 2. An optical pickup according to claim 1, wherein each of saidphotodetector units comprises a photodetector element having saidphotosensitive surfaces thereon, said photosensitive surfaces extendingparallel to each other across said photodetector element in a radialdirection of said information storage medium.
 3. An optical pickupaccording to claim 1, wherein each of said photodetector units comprisesa photodetector element having said photosensitive surfaces thereon,said photosensitive surfaces being arranged successively across saidphotodetector element in a circumferential direction of said informationstorage medium.
 4. A tilt control device for use in an optical pickupfor reading information recorded on a recording surface of an opticalinformation storage medium, comprising:a light beam source for emittinga linear light beam; an optical system including an objective lens forconverging said linear light beam as a linear image on the recordingsurface and collecting and emitting a linear light beam reflected fromsaid recording surface; refracting means for refracting the reflectedlinear light beam emitted from said optical system; a plurality ofphotodetector units each having a plurality of photosensitive surfacesfor photoelectrically converting the reflected linear light beam appliedthereto from said refracting means into a plurality of respectivedetected signals; processing means for processing said detected signalsfrom said photodetector units into a tilt error signal indicative ofwhether said linear light beam is applied perpendicularly to saidrecording surface; and control means responsive to said tilt errorsignal for directly controlling said objective lens positionally withrespect to the optical information storage medium to apply said linearlight beam perpendicularly to said recording surface.
 5. A tilt controldevice according to claim 4, wherein said photodetector units comprisesa first and a second photodetector units for producing a first and asecond detected signals, and said tilt error signal comprises adifference between said first detected signal and said second detectedsignal.
 6. A tilt control device according to claim 5, wherein each ofsaid photodetector means comprises a photodetector element having saidphotosensitive surfaces thereon, said each of said photodetectorelements comprises first, second and third photosensitive surfaces forproducing first, second and third electric signals, and each of saiddetected signals comprises a difference between said first signal andthe sum of said second and third electric signals.
 7. A tilt controldevice according to claim 6, wherein each of said photosensitivesurfaces extending parallel to each other across said photodetectorelements in a radial direction of the optical information storagemedium, and each of said first photosensitive surfaces is disposedbetween said second photosensitive surface and said third photosensitivesurface.
 8. A tilt control device according to claim 5, wherein each ofsaid photodetector element comprises first and second photosensitivesurfaces for producing a first and a second electric signals, and eachof said detected signals comprises a difference between said firstelectric signal and said second electric signal.
 9. A tilt controldevice according to claim 6, wherein said photosensitive surfacesextending parallel to each other across said photodetector element in acircumferential direction of said optical information storage medium.10. A tilt control device according to claim 4, wherein said refractingmeans comprises a flat glass plate disposed on said photodetectorelement.
 11. An optical pickup according to claim 1, wherein saidrefracting means refracts the reflected linear light beam to vary awidth of the reflected light beam in one direction.
 12. An opticalpickup according to claim 11, wherein said one direction comprises adirection perpendicular to a longitudinal axis of the reflected linearlight beam.
 13. An optical pickup according to claim 11, wherein each ofsaid plurality of photosensitive surfaces are arranged in line in saidone direction.
 14. An optical pickup according to claim 1, wherein saidrefracting means is arranged on said plurality of photodetectors in amanner covering the photodetectors.
 15. An optical pickup according toclaim 4, wherein said refracting means refracts the reflected linearlight beam to vary a width of the reflected linear light beam in onedirection.
 16. An optical pickup according to claim 15, wherein said onedirection comprises a direction perpendicular to a longitudinal axis ofthe reflected linear light beam.
 17. An optical pickup according toclaim 15, wherein each of said plurality of photosensitive surfaces arearranged in line in said one direction.
 18. An optical pickup accordingto claim 4, wherein said refracting means is arranged on said pluralityof photodetectors in a manner covering the photodetectors.